Antagonistic Effects of MCPA on Wild Oat (Avena fatua) Control with Diclofop

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Abstract
Experiments were conducted in the field, greenhouse, and controlled environment chambers to determine the extent to which MCPA {[(4-chloro-o-tolyl)oxy] acetic acid} antagonizes wild oat (Avena fatua L.) control with diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid}. Wild oat control with diclofop at 1 kg/ha was reduced from 96% when used alone to 76, 48, 31, and 14% by tank mixture with IAA (3-indole acetic acid), MCPA, 2,4-D [(2,4-dichlorophenoxy)acetic acid], or dicamba (3,6-dichloro-o-anisic acid), respectively. Wild oat control with diclofop applied alone at 1.1 kg/ha was similar to that of diclofop at 2.2 kg/ha applied as a tank mixture with MCPA at 0.15 or 0.3 kg/ha. MCPA antagonism of wild oat control with diclofop increased as the post-treatment temperature increased from 10 to 30 C. MCPA antagonism of wild oat control with diclofop was the same whether the herbicides were applied to the foliage only or to the foliage and soil. Approximately 20% of the wild oat root inhibition with diclofop applied postemergence, however, was from diclofop uptake from the soil. MCPA at 0.6 kg/ha did not reduce wild oat control when applied as a sequential treatment 2 days before or 1 day after diclofop at 1.1 kg/ha.